One way of significantly enhancing the performance of air/liquid heat exchangers is to install one or more spray tubes to spray a liquid onto their heat transfer surface to thereby utilize the latent heat of evaporation of this liquid to boost the heat transfer. The sprayed liquid is vaporized as it passes through the air side flow circuit of the heat exchanger, and the heat removed through evaporation is added to the sensible heat removed by the normal cooling air. This performance enhancement can be used to increase the overall heat transfer capacity of the heat exchanger and/or reduce the cooling air mass flow required while maintaining constant heat transfer performance.
For example, in high performance piston engine powered/propeller driven aircraft where sufficient heat transfer performance is available to cool the engine(s) without evaporative cooling, a very significant reduction in the aerodynamic drag on the aircraft can be achieved with the addition of evaporative cooling by the resulting reduction in the cooling air then required for the heat exchanger(s). This can be a very significant advantage recognizing that the heat exchanger(s) on a high performance aircraft can represent as much as about 50% of the total drag. Water is the liquid that is normally used for the evaporative cooling because of its high latent heat of evaporization. In high performance engine powered/propeller driven aircraft, the engine coolant temperatures are normally maintained between 180.degree. to 220.degree. F. This temperature range is relatively low compared to conventional aircraft and motor vehicle piston engine applications and limits the quantity of water that can be evaporated because of the low temperature difference between the coolant and the atmospheric boil temperature of water, thereby limiting the heat transfer performance gain possible.